1. Field of the Invention
This invention relates to a recording apparatus and more particularly to a recording apparatus of the type having a platen roller and a recording head disposed in parallel and adapted to be pressed against each other and in which a recording paper is passed through a clearance between the platen roller and the recording head, said recording head being more specifically a thermal head.
2. Description of the Prior Art
A recording apparatus that makes a printed or graphic form of recording on a recording paper is employed in various kinds of equipment such as, for example, a typewriter, a facsimile, and a word processor. As one type of such a recording apparatus, there has been widely in practical use a heat transfer type printer wherein a heat-meltable ink coated ink sheet and a recording paper, one placed over the other, are inserted into and passed through a clearance between a roll-form platen and a thermal head, one pressed against the other, so that heat-transfer recording is effected on the recording paper.
FIG. 3 is a schematic view showing a typical arrangement of principal portions of such a heat-transfer printer.
On an ink sheet roll 1 there is wound an ink sheet 2 coated with a heat-meltable ink. One end of the ink sheet 2 is led through a clearance between a platen roll 5 and a thermal head 6 via guide rollers 3, 4, and is then further led through a clearance between a drive roller 7 and pressure roller 8 and is finally wound on a take-up roll 10 via a guide roller 9. As the drive roller 7 is rotated counterclockwise as viewed in FIG. 3, the ink sheet 2 inserted into the clearance between the drive roller 7 and the pressure roller 8 is delivered toward the take-up roll 10, so that the entire ink sheet 2 is sequentially drawn from the ink sheet roll 1 until it is fully wound up on the take-up roll 10. A recording paper not shown is inserted into the clearance between the platen roll 5 and the thermal head 6, or more specifically between the platen roll 5 and the ink sheet 2.
The thermal head 6 is rotatably supported, in a casing not shown, on a supporting shaft 11 extending in the widthwise direction of the ink sheet 2 (which is the same direction as may be sometimes referred to as the widthwise direction of the platen roll 5 and/or the thermal head 6 hereinafter, whereas the term lengthwise direction as may be referred to with respect to the thermal head 6 is the same as the lengthwise direction or passing direction of the ink sheet 2), the thermal head 6 having a width of at that of the ink sheet 2.
At both ends of the widthwise span of the thermal head 6, adjacent to the ink sheet 1, there are provided tension springs in the form of coil springs 12, 12 (only one shown in FIG. 3) which extend to nearer casing walls. Accordingly, the thermal head 6 is biased for clockwise rotation as viewed in FIG. 3 so that a portion thereof nearer to the take-up roll 10 is pressed against the platen roll 5 (along that portion of the thermal head 6 which is pressed against the platen roll 5 there are arranged heating elements in a straight line).
Therefore, by heating the heating elements selectively as the ink sheet 2 and the recording paper (not shown), both inserted in superposed relation into the clearance between the thermal head 6 and the platen roll 5, are moved forward at a constant rate, the heat meltable ink on the ink sheet 2 is transferred onto the recording paper, so that any desired characters, graphics, and the like can be printed thereon.
Now, in a the aforementioned heat-transfer type printer employing a thermal head on which heating elements are aligned over a full widthwise span of the ink sheet 2, it is necessary that the thermal head 6 must, over its full widthwise span, press under uniform force the ink sheet 2 and the recording paper against the platen roll 5. For, if the thermal head 6 on one hand and the ink sheet 2 and recording paper on the other hand are not pressed against each other under uniform force over their full widthwise span, the transfer of heat from the individual heating elements to the ink sheet 2 will not be uniform and accordingly the rate of ink melt at portions of the ink sheet 2 which correspond to the individual heating elements may vary from one portion to another, so that the ink as transferred onto the recording paper may be non-uniform in respect to density. Further, if the thermal head 6 and the platen roll 5 are not uniformly pressed against each other over the full widthwise span thereof, and their mutual pressing force is not uniform in the widthwise direction, the rate of transfer of the ink sheet 2 is likely to be non-uniform from one portion to another in the widthwise direction of the ink sheet 2, and thus creasing is likely to develop on the ink sheet 2. Any creasing developed on the ink sheet 2 may cause of irregularities in ink transfer density on the recording paper. This is true not only with the ink sheet, but also with recording paper, e.g., roll paper and folding paper in particular.
For these reasons, the surface of contact of the thermal head 6 against the ink sheet 2 is flat and the platen roll 5 has a uniform outer diameter so that the thermal head 6 and the platen roll 5 may be brought into a uniform contact condition with each other, by means of biasing, over their full widthwise span. However, when the biasing force of the coil springs 12, 12 is exerted at both ends of the widthwise span of the thermal head 6 as mentioned above, there will develop a slight amount of deformation in the thermal head 6 unless the thermal head 6 is of an ideally rigid construction, and accordingly it is inevitable that the pressing force at the center portion thereof is slightly smaller than that at both ends thereof. Therefore, however high the degree of accuracy taken in designing for flatness of the contact surface of the thermal head 6 with the ink sheet 2 and for uniformity of outer diameter of the platen roll 5, there is still a possibility that a slight amount of deformation may develop in the thermal head 6, which makes it impracticable to expect that the mutual pressing force of the thermal head 6 and the platen roll 5 is uniform over their full widthwise span.
With a view to overcoming aforesaid difficulty that the thermal head and the platen roll may not be brought into complete contact with each other, there have been proposed a few devices, such as those disclosed in Japanese Utility Model Application Laid-Open Nos. 58-92054 (1983) and 58-137652 (1983).
Of these, the device disclosed in Japanese Utility Model Application Laid-Open No. 58-92054 is that "a roller contacts with a thermal head is able to absorb some deformation of the thermal head". However, the "deformation of the thermal head" referred to in the disclosure refers to deformation inherent in each individual unit as caused in the process of manufacture, and not to any deformation due to the biasing force exerted in pressing the platen roll and the thermal head to be pressed against each other. Therefore, above-mentioned device is fundamentally different in concept from the present invention which is intended to solve the problem of deformation caused when a thermal head having its heating elements arranged with high planar accuracy is subjected to biasing forces at both ends at the widthwise span thereof abuts with a platen roll having a uniform outer diameter that is very accurately dimensioned. With the device of Japanese Utility Model Application Laid-Open No. 58-92054, therefore, suitable measures have to be taken according to the quantity of deformation inherent in each thermal head and configuration thereof.
The device disclosed in Japanese Utility Model Application Laid-Open No. 58-137652 is such that "the shaft diametrical size of a platen roll is gradually reduced from a portion adjacent the middle of the length (shaft length) of the roll toward either end thereof and that the outer periphery of the shaft is formed of the same rubber material". Accordingly, the deformation of the rubber in the median portion of the roller along the shaft length thereof is relatively small, and that at both ends of the axial lengthwise span thereof is relatively large; thus, it is claimed that the platen roll and the thermal head can contact each other and have a uniform force exerted therebetween over their entire widthwise span. With this device, however, the shaft of the platen roll must be processed so that its outer diameter is large at its center portion and small at its both ends. Further, it is necessary that the inner periphery of a cylindrical rubber sleeve fitted over the processed shaft must be processed accordingly. Therefore, both the shaft and sleeve must to be processed with high accuracy, which result in a higher product cost.